The present application claims priority based on provisional application No. 60/489,700, filed Jul. 24, 2003, which is hereby incorporated herein by reference in its entirety.
I. Field of the Invention
The present invention is directed generally to improvements in the treatment of aqueous wastewater streams having a content of organic matter such as are collected by municipal sewage systems and, more particularly, to a process which quickly and economically reduces the biological oxygen demand (BOD) in such wastewater to a safe level for discharge back into the environment.
II. Related Art
Wastewater treatment has been and continues to be a matter of great environmental importance. This includes, for example, chemical containing effluents from industrial operations, municipal sewage treatment and treatment of waste from operations producing concentrated animal waste. Traditionally, organic matter containing more than an acceptable or permitted amount of BOD has been treated using microbial action in a manner that includes separation of the organic matter to form a mass of solids known as an activated sludge, with the remaining material making up a water fraction. The treatment problem then becomes decidedly two-fold because the water and sludge fractions both must be treated to be safely returned to the environment and the two fractions may contain different impurities to be removed necessitating different processing.
Treatment typically involves digestion of the dissolved organic material through fermentation of the sludge involving aerobic or anaerobic bacterial action or some combination thereof. These processes are used to reduce or consume the biological oxygen demand (BOD) of the material to an environmentally safe level in the discharged materials. It is also necessary to remove certain undesirable inorganic materials from the water fraction which typically contains undesirable quantities of phosphates and nitrogen compounds including phosphates and ammonia-related compounds.
Systems have been proposed for the aerobic treatment of liquid borne biowastes in which the oxygen potential has been enriched or elevated above that which normally dissolves in the liquid at atmospheric pressure. Such a system is illustrated in U.S. Pat. No. 4,163,712 to Smith in which the gas is introduced into an inlet stream under pressure and turbulent conditions to produce an input stream containing oxygens in amounts greatly in excess of the equilibrium saturation value, the excess forming finely divided bubbles therein. The stream is introduced at or near the bottom of a non-pressurized main biomass allowing the undissolved gas to bubble through the main mass when the pressure is released and to dissolve in the main body of liquid as it rises. This serves to increase the general level of oxygenation in the main reactive biomass, however, in order for such a system to be practical, the oxygen-containing gas mixture needs to have an enriched oxygen content (i.e., greater than that of air) and preferably a composition close to pure oxygen. This, of course, adds greatly to the expense of operating the system inasmuch as it requires a continuous source of liquid oxygen or equivalent to produce the highly enriched air.
Another system which bubbles air through the depth of the biomass is illustrated in U.S. Pat. No. 4,645,603 to Franco in which the air is supplied from an open-ended mixing tube at a pressure slightly above the pressure of the hydraulic head of the tank to be aerated and released to bubble through the reacting biomass.
Additionally, systems and methods have been devised for anaerobically treating wastewater to remove phosphorous and phosphorous-consuming micro-organisms. One such system is shown in U.S. Pat. No. 4,999,111 to Williamson which deals with the removal of phosphates and nitrogen, particularly from wastewater of relatively low BOD to P ratio using both oxic and anoxic zones in which nitrogen, phosphorous and other pollutants are removed. The process requires a fraction of the primary sludge to be fermented to soluble organics to provide significant nutrients to sustain the phosphorous consuming and denitrification systems.
U.S. Pat. No. 5,545,326 to Petering, the inventor in the present application, describes a relatively high pressure process for the treatment of relatively high-solids wastewater having high BOD and phosphorous content which includes both anaerobic and aerobic treatment. That process involves introducing material to be treated at relatively high pressure and processing the material in a closed system prior to rapidly venting the elevated pressure and discharging to atmospheric pressure. That process is more specifically directed to processing wastewater streams of relatively high solids and high BOD content in which phosphorous is removed by being transferred from the liquid to the solid fraction.
Another problem with existing municipal sewage treatment facilities which must handle large volumes of wastewater lies in the fact that they require large, level land areas, sometimes many acres, to accomplish the aeration and separation required to return treated wastewater to the environment. Furthermore, these large sewage treatment facilities are extremely expensive to build, operate and maintain.
Accordingly, there remains a need for a relatively compact, low-cost, high-volume processing system that can return treated wastewater to the environment in a relatively short time. Such a compact system could be employed to treat wastewater on a more local basis as a much smaller site would be required. Modular system units could be added as local demand increased.